The present invention relates to a polishing apparatus and, more particularly, to a polishing apparatus adapted so as to detect a failure during polishing a substrate such as a semiconductor wafer or the like. The failure can include a breakage of the wafer or an event of the wafer jumping from a predetermined position, which may occur during polishing the substrate.
In recent years, semiconductor devices have become so highly integrated that circuit interconnections have become thinner and that distances between these interconnections have also become smaller. Particularly, for an optical lithography of 0.5 μm or less, the depth of focus has become so short that a high degree of flatness on an imaging surface of an exposure apparatus is required. Hitherto, a self-flattening CVD apparatus or an etching apparatus has been used as a flattening apparatus for flattening a semiconductor wafer. These apparatuses, however, do not accomplish a sufficient degree of flatness. Therefore, a polishing apparatus has recently been extensively adopted to flatten a semiconductor wafer because a higher degree of flatness can be expected to be accomplished more readily than the flattening apparatuses described immediately above.
The polishing apparatus of this type is composed of a turntable and a top ring member, each rotating at a number of rotations discretely and independently from each other, and an abrasive cloth is attached on top of the turntable. Between the turntable and the top ring member is interposed a substrate (e.g., a semiconductor wafer), and the substrate is pressed against a top surface of the abrasive cloth at a predetermined pressure. The surface of the substrate is then polished to a predetermined level of flatness and to a mirror surface while a polishing fluid is being fed thereto. After the completion of the polishing process, the substrate is detached from the top ring member and subjected to post-processing processes, including a cleaning process.
It is to be noted herein; however, that the substrate may break and broken pieces may scatter on the abrasive cloth during the polishing process. Damage to the surface of a new substrate results if the abrasive cloth is used again. Therefore, a new abrasive cloth has to be used whenever a substrate breaks. On the other hand, even if a substrate is not broken into pieces, a failure of the substrate may result, for instance, when the substrate jumps from the top ring member during the polishing process. If the substrate is a semiconductor silicone wafer or the like made of a brittle material and the substrate jumps from the top ring member, the substrate may be damaged or chipped at its outer peripheral portion due to impact with a wall surface or the like of the turntable. The damaged substrate is very likely to be broken by application of a slight force onto the damaged portion or at a position close to the damaged portion, when the damaged substrate is polished again.
In order to solve the problems as described above, proposals to prevent a failure while polishing, including a breakage or jump of a substrate from the top ring member, have been devised. For instance, there has been proposed a process for reducing an occurrence of breaking or chipping of a substrate upon polishing the substrate by using a polishing apparatus with a buffer member such as an elastic mat interposed between a bottom surface of the top ring member and the substrate. Alternatively, there has been proposed a process whereby a jump of a substrate is prevented by fixing the substrate within the top ring member while guiding an outer periphery of the substrate.
These proposals, however, are concerned with precautionary measures to prevent a breakage or jump of the substrate, and are useless to deal with an event once the substrate is broken or has jumped outside the top ring member. To deal with an event of a substrate jumping from the top ring member, there has hitherto been adopted a process for immediately suspending a polishing operation of a polishing apparatus as a detection system detects a jump of the substrate outside the top ring member. The detection system is devised to detect the jumping of the substrate from the top ring member and is disposed outside the top ring member.
This process, however, suffers from a problem in that the polishing operation cannot be suspended rapidly because the jumping of the substrate can only be detected after the substrate has already jumped, with the result that the timing of detection is delayed.
Moreover, this process has another problem in that a failure upon polishing caused by a breakage of the substrate cannot be detected. As a result, there is a risk that the polishing of the substrate is continued unless the broken substrate jumps from the top ring member.